1. Technical Field
The present invention relates to optical fiber devices and, more particularly, to cladding-pumped optical fiber lasers and amplifiers.
2. Background Art
Optical fiber lasers and amplifiers are known in the art. In such lasers and amplifiers, rare earth materials disposed in the core of the optical fiber laser or amplifier receive pump radiation of a predetermined wavelength and, responsive thereto, provide or amplify light of a different wavelength for propagation in the core. For example, the well known erbium doped fiber amplifier (EDFA) receives pump radiation having a wavelength of 980 or 1480 nanometers (nm) and amplifies an optical signal propagating in the core at a wavelength in the 1550 nm region.
In such optical fiber lasers and amplifiers, the pump radiation can be introduced directly to the core, which can be difficult due to the small size of the core, or can be introduced to the cladding surrounding the core and absorbed by the core as the rays propagating in the cladding intersect the core. Lasers and amplifiers with the pump radiation introduced to the cladding are known as xe2x80x9ccladding-pumpedxe2x80x9d optical devices, and facilitate the scale-up of lasers and amplifiers to higher power systems.
Absorption per unit length is a useful figure of merit for evaluating a cladding-pumped optical fiber laser or amplifier. It is typically desirable that the amplifier or laser have a high absorption per unit length, indicating that the pump radiation frequently intersects the core. Unfortunately, when the cladding has a circular outer circumference, the pump radiation can essentially propagate down the optical fiber while spiraling around the core without substantially intersecting the core. This leads to a low absorption per unit length of the optical fiber device, and hence detracts from the performance of the optical fiber laser or amplifier.
Various approaches are known in the art for enhancing the intersection of the pump radiation with the core and hence raising the absorption per unit length of the optical fiber amplifier or laser. For example, as disclosed in U.S. Pat. No. 4,815,079, issued Mar. 21, 1989 to Snitzer et al., the core can be offset from the center of the optical fiber so as to enhance the intersection of pump light with the core. In another approach, the inner cladding has a xe2x80x9cDxe2x80x9d-shaped outer circumference that includes a flat section, as disclosed in U.S. Pat. No. 5,864,645, issued Jan. 26, 1999 to Zellmer et al. In another prior art optical fiber, the outer circumference of the cladding is shaped as a polygon, such as a diamond, as disclosed in U.S. Pat. No. 5,533,163, issued Jul. 2, 1996 to Muendel. Other approaches include providing a star-shaped outer circumference of the cladding, as disclosed in U.S. Pat. No. 5,949,941, issued Sep. 7, 1999 to DiGiovanni. See also WO 99/30391, published Jun. 17, 1999, disclosing an optical fiber having a core, inner and outer claddings, and a series of perturbations or irregularities formed in the otherwise circular outer boundary of the inner cladding. The optical fiber is drawn from a preform having rods inserted into holes drilled into the preform for producing the irregularities.
In the foregoing prior art fibers, the non-circular shape of the outer circumference is understood to cause ray distortion and mode mixing of light, thereby directing the light rays of the cladding radiation to the core, and avoiding trapping light in spiral paths that do not intersect the core.
The designs discussed above can have disadvantages. For example, a fiber having an offset core can be difficult to interconnect with other optical components. Designs, such as the diamond and polygon designs discussed above, that require the circumference of the cladding to predominately consist of flat areas, can be difficult to fabricate. The flat areas, which are typically first machined into the preform from which the optical fiber is drawn, tend to deform and change shape when the fiber is drawn at the most desirable temperatures. Accordingly, often the draw temperature is reduced to preserve the desired shape of the outer circumference of the cladding. A reduced draw temperature typically produces optical fibers having higher attenuation and lower mechanical strength. In addition, the star shaped configuration disclosed in U.S. Pat. No. 5,949,941 can be difficult to manufacture. Accordingly, an improved cladding-pumped optical device and/or techniques for manufacturing such optical fiber devices would be a welcome advance in the art.
It is desirable to address one or more of the foregoing disadvantages and drawbacks of the prior art.
According to the preferred embodiment, an optical fiber article for receiving pump radiation of a first wavelength for amplifying or generating radiation of a second wavelength includes a core for propagating light of the second wavelength. The core has a first refractive index and includes a rare earth material. A cladding surrounds the core and has a second refractive index that is lower than the first refractive index. The outer circumference of the cladding includes a plurality of sections, where the plurality of sections includes at least one straight section and one inwardly curved section. An outer layer surrounds the cladding and has an index of refraction that is less than the second index of refraction.
It is considered that the combination of the straight and inwardly curved sections in the outer circumference of the cladding enhances scattering of the pump radiation for more effective absorption of the pump radiation by the core. For example, the inwardly curved section can intercept the pump light reflected from the straight section in a substantially different direction, thus achieving a higher degree of randomization of the paths of the light rays of the pump light for increased interception of the light by the core of the optical fiber article.
Preferably, an optical fiber article in accordance with the invention includes four to twelve sections, where each section of the four to twelve sections is one of inwardly curved and substantially straight. Other sections shaped other than straight or inwardly curved may be present as well. The inwardly curved and straight sections can alternate about the circumference of the cladding. Preferably, each of the inwardly curved sections is spaced from the core of the optical fiber article, at its point of closest approach to the core of the optical fiber article, by a distance that is less than or equal to the spacing between any one of the straight sections and the core at the point of closest approach of any one of the straight sections to the core.
In other aspects of the invention, each of the straight sections is intersected at a substantially perpendicular angle by a different radial vector, and each of the inwardly curved sections are intersected at a substantially perpendicular angle by a different one of other radial vectors. The different radial vectors are spaced by a first angle, and the other radial vectors are spaced by a second angle substantially equal to the first angle. Preferably, the straight sections are longer than the inwardly curved sections.
The optical fiber can be adapted for single mode propagation at the second wavelength, or alternatively, for propagating a plurality of modes at the second wavelength. As is known in the art, in certain fiber designs the core and/or the cladding can be characterized by more than one index of refraction. For example, it is known for the core to have a segmented refractive index profile to broaden the mode fields. Graded index fibers are also known. However, fibers having a core and/or cladding characterized by more than one index of refraction are within the scope of the invention, because for total internal reflection to facilitate guiding light in the core, the cladding includes an index of refraction that is less than an index of refraction of the core, as is well known in the art.
The invention can also include methods practiced in accordance with the teachings herein.
In one aspect, the invention provides a method of making an optical fiber article having an optical fiber core and an optical fiber cladding surrounding the optical fiber core. The method can include the following steps: providing a preform having a preform core and a preform cladding surrounding the preform core, where the preform core includes a rare earth material and has a selected index of refraction, and the preform cladding has an index of refraction less than the selected index of refraction; forming at least one slot in the preform cladding; forming at least one flat area in the preform cladding; and drawing the preform to form the optical fiber article such that the optical fiber article includes an optical fiber core surrounded by an optical fiber cladding having an index of refraction that is less than the index of refraction of the optical fiber core, and wherein the optical fiber cladding includes an outer circumference having at least one inwardly curved section and at least one straight section. In another aspect of the invention, the preform can be drawn at a higher temperature more conducive to providing a lower attenuation and higher strength optical fiber article.
A glass jacket, having an index of refraction that is less than the index of refraction of the preform cladding, can be disposed about the preform cladding and drawn with the preform to provide an optical fiber article having a glass outer layer surrounding the cladding. The glass jacket can be collapsed, such as by heating, onto the preform cladding. The outer circumference of the glass jacket can be shaped, such as to reduce the depth of indentations or depressions in the glass jacket.
In another aspect of the invention, glass soot is deposited on the preform cladding and heated to form a preform outer layer.
The foregoing and other objects, features and advantages of the present invention will be apparent from the following description and accompanying drawings.